Diffuse Degassing Measurements in Geothermal Exploration of Fault Controlled Systems

Abstract

An improvement in surface exploration of fracture controlled geothermal systems shall be achieved by a combined approach of different techniques for diffuse degassing measurements. Gas emissions provide valuable insights into the reservoir characteristics and allow the detection and assessment of permeable fracture zones, which are the preferential targets for geothermal production wells. Areas with high gas emissions can be used as an indicator of major upflow zones from the reservoir through deep reaching, permeable fault zones. The study summarizes two in-situ techniques for the assessment of diffuse degassing processes in geothermal fields from which relative structural permeability has been deduced. This includes spectroscopic measurements of alpha radiation for the determination of the gaseous decay products 222Rn (radon) and 220Rn (thoron), as well as CO2 and H2S measurements by the accumulation chamber method. Both techniques are combined with a method for the determination of electromagnetic gamma radiation. Diffuse degassing measurements are strongly considered to be a successful detection and assessment tool for permeable structural elements. This approach is applied to the Brady’s geothermal system in the Basin-and-Range Province, USA.